1. Field of the Invention
Methods and apparatuses consistent with the present invention relate to color temperature conversion and, more particularly, to color temperature conversion having a luminance correction conversion function, which can represent the gray scale of luminance, occurring when the difference between the color temperature of an image display device and a user-set color temperature is large in the case where the color temperature of an image is converted and corrected to correspond to the user-set color temperature in the image display device.
2. Description of the Related Art
The complete set of hues experienced by a person viewing a scene varies according to the lighting characteristics. For example, under incandescent light, hues are colored red overall, and, under sunlight, hues are colored blue compared to under incandescent light. That is, if a color temperature is high, hues are biased towards blue, whereas, if the color temperature is low, hues are biased towards red. In this case, when hues are related overall to color temperature, the color temperature must be changed so as to change the hues. The term “color temperature” denotes the temperature of light caused by perfect thermal radiation, and is measured in kelvin (K).
A color display system has been widely used in appliances for visually providing information to a user, such as a television (TV), a digital TV (DTV), a thin film transistor (TFT) monitor, a color printer, a digital camera, a projector, or a mobile phone. Owing to the visual characteristics of the color display system, there is a need to precisely calculate the correlated color temperature. The correlated color temperature of light sources denotes the temperature of a blackbody radiator appearing when the light sources have the same color, and is also measured in kelvin. That is, the correlated color temperature represents the wavelength of a light source on the kelvin scale, which is the standard for comparative measurements.
Color models are used to classify colors according to attributes, such as hue, saturation, chroma, lightness and brightness, and specify the classified colors. Color models include a Red, Green, Blue (RGB) model, a Hue Saturation Value (HSV)/Hue Lightness Saturation (HLS) model, a Munsell color system, an International Commission on Illumination (CIE) color model, etc. The CIE color model was defined by the International Commission on Illumination (CIE) for establishing standards for illuminators. A CIE color model includes a CIE_XYZ color model, a CIE_LUV color model and a CIE_LAB color model. The CIE_XYZ color model represents RGB tristimulus values as an XYZ, which is a set of different tristimulus values, each having a positive polarity. The CIE_XYZ color model uses a chromaticity diagram.
Meanwhile, a related art image display device converts the color temperature of an image reproduced on an image display device by adjusting the hue or the amount of RGB or by performing a matrix operation. Such a related art color temperature conversion method influences the entire content of a reproduced image. This method is problematic in that, if the difference between the target color temperature of an image to be reproduced and the basic color temperature of the device is large, a phenomenon in which the image, obtained after conversion, seems to have been passed through a specific color filter, occurs, and the naturalness of the reproduced image is deteriorated due to the degradation of color in faces, easily perceived by persons.
A related art for solving this problem is a “color temperature conversion method and apparatus having a correction function according to the luminance of an image pixel” disclosed in Korean Patent Laid-Open Publication No. 10-2005-0087266, previously filed by the applicant of the present invention. FIG. 1 is a flowchart illustrating this related color temperature conversion method having a correction function according to the luminance of an image pixel.
The related art color temperature conversion method and apparatus convert a current color temperature into a color temperature other than a color temperature provided by an image reproduction device, or reproduce different color temperatures for respective luminance values of input pixels in consideration of users' preferences. With reference to FIG. 1, the related art color temperature conversion method is described. First, if a YCbCr image signal is received at operation S10, a variable for determining the conversion object range, which is a quadratic curve, is determined at operation S20. Further, whether an input pixel is a conversion object is determined by determining whether an input pixel exists within the conversion object range based on the determination variable at operation S30. If it is determined that the input pixel is a conversion object, a user-set color temperature is received, and is compared to the basic color temperature at operation S40. If the user-set color temperature is different from the basic color temperature as a result of the comparison, conversion target chromaticity coordinates corresponding to the user-set color temperature are calculated with reference to a Look-Up Table (LUT) containing chromaticity coordinates corresponding to a color temperature for each sample luminance at operation S70. If the user-set color temperature is equal to the basic color temperature, conversion target chromaticity coordinates corresponding to the basic color temperature are calculated with reference to an LUT containing basic color temperature coordinates for each luminance at operation S50. Further, the chromaticity coordinate conversion operation, of converting the chromaticity coordinates of the input pixel to correspond to the extent to which an origin on a chromaticity coordinate plane is moved to the conversion target chromaticity coordinates, is performed at operation S60. Finally, it is determined whether all pixels have been processed at operation S80. If it is determined that any pixel remains to be processed, the process returns to operation S20, whereas, if it is determined that there are no remaining pixels, the process is terminated.
The related art method and apparatus selectively perform color temperature conversion and correction with respect only to a gray region in a color space. Therefore, even when the difference between the basic color temperature of the device and the user-set color temperature is large, degradation of other colors does not occur, and color temperature conversion is performed for each luminance. Accordingly, color temperature control for each luminance to satisfy a user's preference is possible, and the capability of reproducing a gray color is improved.
However, when the difference between the color temperature of an image display device and the user-set color temperature is large, the difference between the luminance values of a selected gray region and the remaining regions increases, thus causing a problem related to gray scale representation in that pixels having different luminance values in an input image are represented by the same luminance in an output image. Such a problem related to gray scale representation is shown in FIG. 2. FIG. 2 illustrates the case where a point placed outside a YCbCr color space is mapped to a point placed inside the color space according to the color temperature conversion method of FIG. 1. Referring to FIG. 2, when a certain color temperature is converted into a user-set color temperature in a gray region, given points A and B are moved to A′ and B′, respectively. In this case, the point A′ exists outside the color space, and thus the color of the point A′ cannot be represented. Accordingly, in the prior art, a method of resealing colors, which exist outside the color space after color temperature conversion has been performed, on the basis of maximum values in an RGB color space, and mapping the rescaled colors to colors that exist inside the color space, is used. This operation is expressed by the following Equation 1.If Max(R,G,B)>255,R′=R/Max(R,G,B)*255G′=G/Max(R,G,B)*255B′=B/Max(R,G,B)*255  (1)
However, such a related art method is problematic in that, when original colors placed on an A-B line are converted into colors placed on an A′-B′ line through color temperature conversion, all points on the A-B line are mapped to the single point B′ existing in the color space, as shown in FIG. 2, so that distortion occurs in the representation of gray scale values corresponding to luminance. That is, there is a problem in that colors, originally having different luminance values, are converted into a common point having the same luminance after color temperature conversion has been performed.